The Union of Democratic Forces (Bulgarian: Съюз на демократичните сили, Sayuz na demokratichnite sili, СДС, SDS) is a political party in Bulgaria, founded in 1989 as a union of several political organizations in opposition to the communist government. The Union was transformed into a single unified party with the same name. The SDS is a member of the European People's Party (EPP).

Dissident groups formed under the faltering regime of Todor Zhivkov in the late 1980s were the basis for the Union. Once Zhivkov fell, a loose political confederation was envisioned where constituent groups could continue to work for their own cause, while the coordinating council would include three members from each organization. The Longtime dissident philosopher Zhelyu Zhelev, who would later become Bulgaria's President, was elected chairman, and Petar Beron, a well-known environmental scientist, was chosen as secretary.

However, following discontent over economic problems, Prime Minister Zhan Videnov resigned at the end of 1996, clearing the way for new elections, this time won by the SDS with a crushing 55% vote. Party leader Ivan Kostov went on to form the new government and successfully passed several economic reforms. He was eventually rewarded in December 1999 with an invitation to begin membership talks with the European Union. But public discontent over the social cost of the reforms, including increased unemployment, as well as allegations of corruption led to the SDS's defeat in the June 2001 elections, which were won by the National Movement for Simeon II. The United Democratic Forces won 18.2% of the popular vote and 51 out of 240 seats.

The SDS was chaired until the May 20, 2007 European elections by Petar Stoyanov, former president of the country. Meanwhile, Kostov, the former Prime Minister and SDS party leader went on to form his own party - Democrats for a Strong Bulgaria. At the 2005 parliamentary election, the United Democratic Forces won 8.4% of the popular vote and 20 out of 240 seats.